1. Technical Field
The present invention relates to a thermal transfer printer, and particularly to a technique to suppress lateral banding from occurring in a thermal transfer printer that performs multi-screen printing such as two-screen printing or three-screen printing.
2. Related Art
In a thermal transfer printer, such as a dye sublimation printer, it is preferable to use an ink ribbon having an optimal length for a given print size. For example, to print images having sizes of 6×4 inches and 6×8 inches, it is preferable to use dedicated ink ribbons for the respective sizes. There is known a technique for performing two-screen printing by which consumption of materials and a printing time are reduced. In the known technique, an ink ribbon having a size of, for example, 6×8 inches may be used to perform two-screen printing of two images each having a size of 6×4 inches, in a single process. There is also known a technique of performing three-screen printing by using an ink ribbon having a size of 8×12 inches to perform three-screen printing of three images each having a size of 8×4 inches, in a single process.
FIG. 1 shows a structure of an image forming unit in a thermal transfer printer. Dye that is coated on an ink ribbon 120 is heated by a thermal head 130 and transferred onto a print sheet 141. The ink ribbon 120 and the print sheet 141 after completion of the transfer are separated from one another by a peeler plate 131. A load necessary to effect peeling (hereinafter, “peel force”) of the ink ribbon 120 from the print sheet 141 varies depending on an image to be printed. In general, a higher a density of an image to be printed, a greater a peel force required.
In a case of printing plural screens, as noted above, a roll sheet is used and printing is carried out such that a blank space is provided between adjacent screens so that a printed screen is not influenced by another screen. This blank space is cut out finally upon completion of printing, and as a print result only the printed screens are output. In this case, no images are printed on the blank space, and therefore a peel force at the blank space varies greatly from that at other parts where images are formed. This kind of variation in peel force (hereinafter “load variation”) causes a tension of the ink ribbon 120 to vary. Such variation in tension of the ink ribbon 120 results in lateral banding in printed images.
Techniques for preventing lateral banding are described in, for example, Patent documents 1 and 2. Patent document 1 discloses a technique for preventing lateral banding by increasing a tension of an ink ribbon; and Patent document 2 discloses a technique for applying a bias energy to a blank space in an ink ribbon (e.g., energy at a level which does not give rise to coloring) so as to prevent lateral banding.
Patent document 1: JP-A-8-197762
Patent document 2: JP-A-7-125293